Small and compact printed circuit assemblies (PCA) with miniature surface mount technology (SMT) components and custom application-specific integrated circuits (ASICs) installed on dual-sided, multi-layer printed circuit boards (PCBs) are now common. The spacing between the pins of the components becomes smaller as the designs are made to fit into smaller physical configurations. The physical spacing, such as pin spacing and wire trace spacing, is further reduced when the assembly is intended to be portable, such as an assembly for a modem designed to support the Personal Computer Memory Card International Association (PCMCIA) standard.
It is often difficult to determine if a component has been installed correctly in such a crowded and densely populated PCA. More particularly stated, it is often difficult to determine the integrity of the electrical connection between the wire traces of the PCB and the pins or leads of the installed component. The component may be an electronic device or a connector. One method of testing a PCA involves the use of a “bed of nails” test fixture with a conventional in-circuit tester, such as a 3070 Board Tester manufactured and distributed by Agilent Technologies of Santa Clara, Calif. The bed of nails test fixture provides a number of contact probes for accessing test points on the PCA. For the test method to work, there must be contact between the probes of the fixture and the component or signal paths on the PCA. The method is often called a “bed of nails” testing method because the probes are typically sharp metal contact probes configured so that the PCA can be placed on the “bed of nails” and tested. In this manner, the probes touch or access various parts of the component or the signal paths on the PCA and thereby allow measurements to be made.
Typically, the component is stimulated through signals provided through the probes. Measurements obtained from the component via other probes are then compared to “correct” values to determine if the component on the PCA is installed correctly. However, the test probes must be in contact with the appropriate signal paths for this testing method to work. Densely populated printed circuit assemblies often have inaccessible signal paths, such as wire traces beneath multiple layers on the printed circuit board assembly, thereby hampering the use of such a “bed of nails” test fixture. Therefore, “bed of nails” test fixtures are often ineffective when attempting to test a densely populated PCA.
A non-contact testing method for testing the integrity of a device's connections is capacitive testing. An example of capacitive testing is disclosed in U.S. Pat. No. 5,254,953; Crook et al., entitled “Identification of Pin-Open Faults by Capacitively Coupling through the Integrated Circuit Package”. In this patent, a system is disclosed for determining whether pins of an integrated circuit (IC) device are properly soldered to a printed circuit board (PCB) of a printed circuit assembly (PCA). A capacitive sensor is positioned over the IC device while a test probe contacts a pin under test via a pad and a connection between the pin and the pad. A 0.2 volt 10 kHz alternating current (AC) test signal is injected via the test probe into the pad connected to the pin under test. The capacitive sensor then detects this test signal via the capacitive coupling between the pin and the bottom of the capacitive sensor. The capacitive sensor converts the AC signal to an intermediate signal called a detection signal, by low-pass filtering the AC signal. The value of the detection signal is proportional to the detected amplitude of the AC signal. In this manner, the value of the detection signal from the capacitive sensor may be compared to a threshold value to determine characteristics about the detected AC signal (such as the strength of the AC signal). If the electrical connection between the test probe, the pad, and the pin under test is open, the value of the detection signal will be much smaller than anticipated. An in-circuit tester (not shown) connected to the capacitive sensor then indicates that the PCA has failed the test and declares that the pin under test is open.
In capacitive testing, several cycles, for example five cycles, of the 10 kHz analog test signal is required to test the integrity of one pin. That is, it takes as long as 500 μsec to test one single pin. Moreover, probe access to the pin under test is still required to apply the test signal for such capacitive testing. Thus capacitive testing would still be ineffective in a circuit assembly where test probe access to the pin under test is not available, such as a densely populated circuit assembly where the pin spacing and wire trace spacing are extremely small.
U.S. Pat. No. 6,104,198; Brooks, entitled “Testing the Integrity of an Electrical Connection to a Device Using an Onboard Controllable Signal Source” discloses the use of an on-board microprocessor or BSCAN device as a controllable signal source for sequentially applying a 10 kHz signal to each pin under test of a device. This solution at least partially eliminates the access problem. However, testing throughput may still be a concern, especially if there are a large number of pins to be tested. Furthermore, the need to generate a 10 kHz signal limits the application of this technique.